What precautions should my sister take for skin allergy?

Your sister seems to have photosensitivity. Various kinds of skin lesions appear in different people. Which exact kind of photosensitive disorder she has, can only be found out after clinical evaluation. Photosensitive disorders can result from different mechanisms; it could be genetic, auto immune, or due to photosensitising chemicals present in the environment, etc. Some disorders may require specific therapy. However, in all cases, protection from light by appropriate clothes and sunscreens, will give good relief.